A thrust disc equipping a magnetic bearing usually rotates at high speed, for example between 300 and 400 meters/second, and is subjected to high stress. In corrosive environments, in particular including hydrogen sulphide (H2S), such thrust disc must comply with norm NACE MR0175, have high mechanical properties and high magnetic properties at the same time.
For example, the mechanical and magnetic properties of a specific material can be quantified by its offset yield strength “Rp0.2” and its magnetic permeabily “μ”, respectively. In mechanics, yield strength Rp0.2 defines a relation between a plastic deformation and a stress applied to a material. In electromagnetism, permeability μ is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, permeability corresponds to the degree of magnetization that a material obtains in response to an applied magnetic field.
WO-A-2013/113678 discloses a thrust disc, comprising an annular bush, a body and a magnetic core. The bush is mounted on a shaft belonging to a turbomachine rotor. The body is fixed to the bush and enclosed the magnetic core. Depending on its material, the body may be resistant to corrosion but increases the magnetic airgap. Moreover, the complete hermetic sealing of the disk, on all its faces, is highly complicated to achieve. Furthermore, at a high pressure such as approximately 100 bars, a leak may appear in the body, so that corrosive gas may enter inside the cavity housing the magnetic core during the lifetime of the thrust disc. This leak will imply a possible corrosion of the magnetic core, internal pressure accumulation and high risk of explosive decompression.